Wireless networks have significantly impacted the world in the last debacles and their uses continue to grow significantly. People and businesses use wireless networks to send and share data quickly whether it be in a small office building or across the world. Emergency services such as the police department utilize wireless networks to communicate important information quickly. Another important use for wireless networks is as an inexpensive and rapid way for connection to the Internet in countries and regions where the telecom infrastructure is poor or there is a lack of resources, like in many developing countries. One of the latest such wireless technologies is WiMax (Worldwide Interoperability for Microwave Access).
A brief review of the WiMax technology follows for providing the readers with a better understanding of the invention. It is to be noted that the invention is not limited to the WiMax technology, but it is applicable to any wireless technology that uses predetermined periodic bit sequences and multiple antennas in downlink and uplink frames, such as frame preambles, pilot tones, and/or ranging codes used by WiMax.
WiMax is an emerging telecommunications technology that provides long range wireless communication, and enables both point-to-point and full mobile cellular type access. This technology is based on IEEE 802.16 standard. The initial draft standard for this technology, called 802.16d, or 802.16-2004 has never reached the standard status. Systems built using 802.16-2004 (802.16d) and OFDM PHY with 256 carriers as the air interface, are generally referred to as “fixed WiMax”.
The next version of the draft, 802.16e (or 802.16-2005), which is an amendment to 802.16d, is often referred to as “mobile WiMax”. This term refers to wireless systems that use 802.16e-2005 and OFDMA (orthogonal frequency-division multiple access) with 128, 512, 1024 and 2048 carriers as the air interface. In OFDMA, a spread-coded string of symbols of a signal to be transmitted is modulated on subcarriers which are preferably distributed into a broad frequency band. OFDMA assigns subsets of subcarriers to individual users, and based on feedback about the channel conditions, the system can implement adaptive user-to-subcarrier assignment.
Mobile WiMax implementations can be used to deliver both fixed and mobile services. The mobile WiMax also uses Multiple Antenna Support through Multiple-Input Multiple-Output communications (MIMO). A base transceiver station (BTS), also called a base station (BT), uses at least two receiving antennae and two transmitting antennae and the user equipment (UE unit) uses at least two receiving antennae and a transmitting antenna. This brings potential benefits in terms of coverage, spatial diversity and spatial multiplexing, interference cancelation, frequency re-use and spectrum efficiency.
Mobile WiMax has just been approved by ITU, and telecommunication companies such as Sprint-Nextel in USA and France Telecom in France have announced their intention to deploy such systems. In Canada, Rogers communications and Bell Canada started to provide WiMax based Broadband Internet service on 2.5 GHz frequency band, covering most major cities like Toronto using Motorola's DRM units.
In the meantime, the advancements in the wireless networks technologies enabled deployment of wireless location positioning systems, particularly systems designed to locate the geographical position of callers that place emergency calls (such as “911” in USA and Canada) using a mobile device. One of the purposes of this service is to enable a wireless ‘network to identify to which Public Safety Answering Point (PSAP) to route an emergency call and to inform the PSAP that answers the call where the caller is. A PSAP will then exploit the knowledge about where a caller is located and provide the information of his/her surroundings such as directions, nearby restaurants, museums, etc to the emergency services. Location based services have been a hot topic for B3G (beyond 3G) wireless systems—such as 3GPP/UMTS/LTE (long term evolution), WiMax/IEEE 802.16e, UMB (ultra mobile broadband) etc.
Currently, the “911” service is capable of locating fixed phones in most geographical areas in the United States and Canada; other countries have similar emergency services. For wireline “911”, the location is an address.
The U.S. Federal Communications Commission (FCC) rolled out a location technology called E911 (Enhanced 911), which enables cellular/mobile devices to process 911 emergency calls for timely deployment of assistance. For Wireless E911, the location is a coordinate. The FCC has rolled out E911 in two phases. In 1998, Phase I required that mobile phone carriers identify the originating caller phone number and the location of the signal tower, or cell, with an accuracy of less than one mile. In 2001, Phase II required that each mobile phone company doing business in the United States must offer either handset or network-based location detection capability so that the caller's geographic location, termed ALI (Automatic Location Identification) be provided with an accuracy of less than 100 meters.
Several methods are known for determining the location of a mobile caller (MC) as required by Phase I. These are called “network based” methods since they employ a wide area array of antennas and transceivers coupled together, and a mobile caller can be located whenever contained within the area that is covered by the respective transceivers/antennae. Such methods usually require minimal modifications in the mobile devices involved in ALI. However, the current network based methods are not very accurate and may not work particularly well in an indoor environment.
For example, it is known to measure the Angle of Arrival (AOA) of a signal received at two (or more) base station antennae; trigonometric calculations then establish the caller's coordinates using the known location of the antennae and the AOA of the received signal.
It is also known to identify the location of a MC by measuring the Time of Arrival (TOA) of a signal emitted by the caller's mobile at three (or more) network antennae. The location of the MC can be then determined knowing the location of these antennae, the three TOA's measurements, and the velocity of the signal (the velocity of electromagnetic waves/light). This is accomplished by determining the geometric locus of the points at a fixed, known distance (range) from a fixed point (the location of the MC); the range is determined from the TOA. As this method gives two points, a fourth antenna is used sometimes to remove this ambiguity or to compensate for clock discrepancies.
Other network based solutions provide the location of the mobile by measuring at a base station the round-trip delay of a signal sent from the base station to mobile and back, or in other words, the time elapsed between transmission of a signal from the base station and reception of the response from the mobile. This round trip delay is then used to evaluate the distance between the two; the distance and the AOA measurement at the base station are used to estimate the coordinates of the mobile.
However, the AOA, TOA and round trip delay methods are based on line of sight distance measurements (straight distance between the UE unit and the antennae), which can be difficult or impossible to determine in mountainous terrain or in the cities around high buildings and other obstacles. Therefore, the results obtained with these methods are inaccurate. In addition, the location of the caller is not very accurate, especially in the case of indoor calls.
Currently, Phase II of the E911 technology is mainly implemented using Global Positioning System (GPS) embedded into the caller's equipment. The GPS units are embedded in the mobile devices and normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS satellites (i.e. GPS/NAVSTAR). These satellites transmit both satellite positioning data and GPS-assist data; such as clock timing or “ephemeris” data. If the roaming device is known to be essentially on the ground (e.g., mounted in a car), the earth globe, with proper topography, can be used as an additional reference “sphere” to refine the TOA calculations.
However, the process of searching for and acquiring GPS signals, reading the ephemeris data for a multiplicity of satellites and computing the location of the receiver from this data is time consuming, often requiring several minutes. In many cases, this lengthy processing time is unacceptable, particularly in emergency situations where location is being determined for a 911 dispatch centre. In addition, in order to use GPS, the mobile device must be GPS-enabled, which is not always the case. Equipping the mobiles with GPS units also increases the cost, which may become prohibitive for many. Still further, a GSP receiver does not operate properly in some types of environment such as indoors or where satellite signals get blocked.
All the methods described above have not yet provided satisfactory solutions to the problem of wirelessly determining the location of callers using small, inexpensive and low power roaming devices. Also, current methods and systems do not operate well over a wide area, without requiring a dedicated infrastructure.
Therefore, a need to improve location determination still exists, both with a view to enhance the services offered to mobile device users and particularly in with of the E911 regulations by the FCC in the US.